iv MUSCULAR SYSTEM 63 



and finally to join the spinal cord, which, as we have seen, 

 is in connection with the brain. In the living frog nervous 

 impulses originate in one or other of these organs, without 

 the direct intervention of an external stimulus, and are con- 

 ducted along the nerves to the muscles. But further con- 

 sideration of this subject must be deferred until we have 

 made a special study of the nervous system. 



The Muscular System in General. — All over the body 

 the muscles, though varying greatly in form — some being 

 elongated and band-like (Fig. 16, sar), others spindle-shaped 

 igastr), others in the form of broad flat sheets {my. hy, obi. 

 exf)- — have the same general relation to the skeleton as in 

 the case of the gastrocnemius. Each muscle arises or has 

 its origin in a relatively fixed part, and is inserted into a 

 relatively moveable part. As each muscle contracts in one 

 direction only, it follows that the more complex the move- 

 ments any part is capable of performing, the more numerous 

 must be its muscles. For instance, the femur, which, as we 

 have seen, is capable of universal movement, has no fewer 

 than nine muscles, arising from various parts of the hip-girdle, 

 inserted into it. Even the minute phalanges of the fingers 

 and toes all have their little slips of muscle by which the 

 various movements of grasping and relaxing, approximating 

 and separating the digits are effected. 



There are certain terms applied to muscles which it is useful to know. 

 A muscle which raises a part, e.g., the lower jaw, is called a levator, 

 one which lowers a part a depressor. A muscle which serves to straighten 

 one part upon another, e.g. , to bring the shank into line with the thigh, 

 is an extensor, one which bends one part on another is a flexor. A 

 muscle which draws a limb towards the trunk is an adductor, one which 

 draws it away anabdzictor, one which rotates one part upon another, e.g., 

 the femur on the pelvis, a rotator. 



Thus all the complex and accurately adjusted movements 

 of the frog are performed by the contraction of its numerous 



